1. Field of the Invention
The present invention relates generally to telecommunications and more particularly to transmitters and receivers for fiber optic networks.
2. Background Information
In current fiber optic networks, an electronic data stream is fed to an optical fiber multiplexor, which is also called “a box” in the industry. Each multiplexor runs on a specific transmission standard, for example, SONET. A laser and an amplitude modulation circuit for the laser typically are located on a card, which fits into the box. The laser amplitude modulator typically pulses or alters the laser output to create an amplitude-modulated optical signal representative of the electronic data stream. The laser amplitude modulator and laser thus define a transmitter for transmitting the optical signal over an optical fiber. A receiver for the amplitude-modulated optical signals of the optical data typically includes a photodiode to convert the optical signals back into the electronic data stream. Both the transmitter and the receiver typically are located on the backplane of a single card, which is replaceable should a component fail.
The card typically also contains a connector for receiving at least one optical fiber, for example a duplex SC connector. The connectors normally are located on a faceplate of the card, the faceplate being perpendicular to the backplane.
The reading of the amplitude-modulated optical data signals using the photodiode on the card is straightforward: the optical signals either produce an electric output at the photodiode or they do not. As a result, an output electronic data stream of zeros and ones is generated.
The electronics for the amplitude modulation of the laser and for the receiving of the optical data on the card thus is relatively simple. All that is required is a pulsing circuit for pulsing the laser as a direct function of the input data and a photodiode for the receiver.
Existing amplitude modulated systems have the disadvantage that the fiber can be easily tapped and are not secure.
U.S. Pat. No. 5,455,698 purports to disclose a secure fiber optic communications system based on the principles of a Sagnac interferometer. A data transmitter is a phase modulator for modulating counter-propagating light beams sent by a receiver round a loop. The receiver includes a light source, a beamsplitter for splitting light from the light source into counter-propagating light beams and for receiving the phase-modulated light beams, and an output detector. U.S. Pat. No. 5,223,967 describes a similar Sagnac-interferometer-based system operating over a single optical fiber.
The Sagnac-interferometer-based systems described in these patents have the disadvantage that they require the light to travel over a loop, whether back and forth in a single fiber or over a long length looped fiber. As a result, either the link budget for the single fiber must be doubled, reducing the data carrying capacity for a single fiber, or else a looped fiber with significant and expensive extra length of at least twice that of a single fiber must be laid between the transmitter and the receiver. Moreover, the receiver contains the light source, as opposed to the current installed base where the transmitter has the light source.
The Sagnac-interferometer-based systems thus are expensive to build and operate, and do not work particularly well with existing multiplexors or card formats.
U.S. Pat. No. 6,072,615 purports to describe a method for generating a return-to-zero optical pulses using a phase modulator and optical filter. The RZ-pulse optical signal transmitted over the fiber is easily readable by a detector. No card is disclosed. The return-to-zero signals also are read by a photodiode.
U.S. Pat. No. 5,606,446 purports to describe an optical telecommunications system employing multiple phase-compensated optical signals. Multiple interferometric systems are combined for the purpose of multiplexing various payloads on the same optical transmission path. The patent attempts to describe a method for providing fiber usage diversity using optical coherence length properties and a complex transmit/receive system. Each transmitter has a splitter, a plurality of fibers and a plurality of phase modulators to create the multiplexed signal, which is then demultiplexed at the receiver. This system is complex and expensive. Moreover, the interferometers disclosed are not part of a card-based system, as the patent only mentions that the tributary cards for producing electrical signals are card-based, as disclosed at column 14, lines 52 et seq. The system of the '446 patent is thus difficult to implement with existing commercial boxes.